I have been researching cold plasma for a project. I am trying to write a diagram on the temperatures of cold plasma but I can’t find any good resources on the temperature range of the plasma. So my question is how cold can cold plasma get and how do you make it colder?
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$\begingroup$ I know the plasma in the outer heliosphere is down below ~1 eV (i.e., less than 10,000 K) but I am not sure how much cooler it gets because the instrumentation was not meant to resolve such cold particle distributions. In principle, one could go much lower so long as things stay quasi-neutral but I am not sure what the lower bound is as the recombination rates aren't always simple functions of temperature... $\endgroup$– honeste_vivereCommented Dec 18, 2023 at 21:53
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A fluid undergoes a phase transition to plasma when ionized particles become long-lived enough to contribute meaningfully to the dynamics of the fluid. In a hot plasma you might accomplish this by making the mean kinetic energy comparable to the ionization energy of one or more species. But you can also have cold plasma at low pressure, if the mean free path for ions to collide and recombine is very long.
A "cold cathode vacuum gauge" only works below the plasma transition pressure in a vacuum system. They work in room temperature vacuum chambers.
Many years ago I saw a nice plot of the plasma transition curve as a function of temperature and pressure. I have looked for it occasionally without success.
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$\begingroup$ As you know, there are many space plasmas well below the ionization temperature of any of the constituents. Presumably these cooled to these temperatures, not started as such. Given that the thermal pressures in space plasmas can be 12 orders of magnitude lower than our best lab vacuum chambers, I wonder how far we can extend thermodynamic concepts and predictions? $\endgroup$ Commented Jan 2 at 15:07
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1$\begingroup$ Note that a "neutral" gas will always have some transient ionization due to interactions with non-thermal cosmic radiation. Below the plasma transition curve in temperature/pressure space, that ionization stops being transient, whether the gas has ever been hot or not. $\endgroup$– rob ♦Commented Jan 2 at 17:04